This invention relates to the formation of an enclosure, capable of retaining pressure, especially for environmental sealing of substrates such as splices in pressurized cables.
Although the invention is applicable to the formation of other pressure vessels it will be described primarily in term of the formation of a pressurized splice case for a telecommunications cable.
A cable splice must be protected against the environment if the supply lines within it (for example electrical conductors) are to function over their intended life-time. A major cause of failure in telecommunications cables is due to water or moisture vapour in contact with the conductors, and such water or vapour is most likely to enter the cable at points where it has been spliced or repaired. Splice cases and repair sleeves must therefore provide a very effective environmental seal over a long period of time. There is a further requirement, however, in respect of some of the larger cables. Main telecommunications cables are generally pressurized with dry air from the exchange, firstly to prevent ingress of water vapour (or other contaminants) and secondly to provide an early indication, by means of a localized pressure drop, of where and when a leak has occured.
Pressurization of cables puts a further requirement on the design of splice cases, repair sleeves and terminations etc: the enclosure around the cable must engage the cable in such a way that the engagement will not fail due to the pressure within the enclosure.
A seal between the cable and the enclosure is generally provided by an adhesive bond. However, for the purposes of the present invention, a sealant such as a mastic with little or no adhesive properties, or a mere mechanical force between cable and enclosure will be sufficient. This seal, like any such seal, is most susceptible to failure by peel, a greater force being required to separate surfaces by a sheer force for example. Pressure within the enclosure may unfortunately put the seal into peel.
Attention has therefore been directed to ways of sealing the enclosure to the cable such that internal pressure does not cause peel. One solution is to provide the enclosure with specially formed end pieces which have what is known as an inside-out configuration: thus, the ends of the enclosure do not simply taper down onto the cable forming an actuate angle with it, but instead are turned in on themselves such that the outer surface of each end piece seals to the cable. In this way, pressure within the cable tends to reinforce the seal.
A disadvantage of this solution is the cost of such end pieces and the fact that any given end piece is limited to a very narrow range of cable sizes and can not in general be used around cable branch-offs.
The desire for simplicily of installation and reduction of inventory and cost, has led to the use of simple recoverable sleeves, particularly wrap-around sleeves, for the formation of splice cases. Although such sleeves may be manufactured with a shape having regard to the shape or configuration of splice to be enclosed, an inside-out configuration will generally not be provided.
The sleeves are preferably heat-shrinkable being made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Pat. Nos. 2,027,962; 3,086,242 and 3,597,372. As is made clear in, for example, U.S. Pat. No. 2,027,962, the originally dimensionally heat stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded whilst hot, to a dimensionally heat-unstable form, but in other applications a preformed dimensionally heat stable article is deformed to a dimensionally unstable form in a separate stage.
In other articles, as described for example in UK Pat. No. 1440524 (equivalent to U.S. Pat. No. 4,035,534), an elastomeric member such as an outer tubular member is held in a stretched state by a second member, such as an inner tubular member which, upon heating, weakens and thus allows the elastomeric member to recover.
UK Pat. No. 2113925 (equivalent to U.S. Pat. No. 4,466,846) discloses a basic solution whereby such sleeves may be made pressure retaining over the long periods of time required. A sealing strip is provided which can be wrapped around the cable and over which a recoverable sleeve is recovered, such that the strip is positioned at a junction between the cable and the sleeve. The sealing strip, in a preferred embodiment, has a V-shaped cross-section, one arm of the V becoming bonded to the sleeve and the other to the cable. Pressure within the sleeve can cause the V to open out. It can be seen that the junction between the cable and the sleeve is not put in peel by internal pressure, due to interposition of the sealing strip. It may be noted here that bond between an arm of the V and the cable (and between the other arm and the sleeve) is not subjected to peel by internal pressure, but on the contrary may be re-inforced by the pressure. Thus, even if the distal end of that arm were not bonded to the cable there would be no tendency for the bond between the remainder of the arm and cable to fail, the reason being that equal pressures act against each surface of the end of that arm.
Where an enclosure extends around the entire periphery of the substrate (as in the case of a sleeve around a cable) then the sealing strip must likewise extend around the entire periphery of the substrate if the entire seal requires protection against peel. This is clearly not a problem if the sealing strip has the form of an annulus. If, however, the strip has ends (which is desirable since it allows the strip to be wrapped-around the substrate rather than slid over a free end of the substrate) then consideration must be given to pressure retention by the strip at its ends. In an embodiment of GB No. 2113925 the two arms of the V are bonded together at each end of the strip. The strip may thus be wrapped around the cable with its ends overlapping, pressure being retained between the arms of the V by its closed ends.